2 Outputs System Components Connecting to a NPN PLC NPN PLC inputs are normally pulled to high inside the PLC. Closing camera opto connects the PLC input to Ground through the camera opto making the PLC signal go low. The camera outputs should be set to Active Low so that when camera output is active (opto open), the PLC is pulled up to +V through its internal pull-up resistors. HAWK MV-4000 Smart Camera Guide 2-25